The accurate measurement of specular reflectance over a range of wavelengths from ultra violet through near infrared and over a range of angles of incidence, is a prerequisite to the design and manufacture of a variety of modern optical components.
In spectrometers the light source and the light detector are usually fixed in position. The specular reflectance apparatus diverts the light beam onto a sample of interest and re-orients the reflected light onto the detector. This involves use of a number of mirrors that redirect and refocus the light beam to allow for changes in the length of the path between the light source and the detector. The mirrors are usually fixed and/or movable in position. The need for a plurality of mirrors makes such apparatus complex. Moreover the mirrors tend to deteriorate with handling and exposure to the atmosphere, progressively degrading the overall performance of the apparatus.
Measuring absolute specular reflectance as a function of the angle of incidence of the light beam avoids the need for multiple mirrors but requires considerably more complex optical arrangements than conventional spectrometers. The principle requires two measurements to be made, that is a first reference measurement excluding the sample and a second measurement that determines the reflectance of the sample.
One suitable arrangement involves switching between the reference and sample measurement configuration by rotating a mirror and translating the detector assembly automatically. The mirror rotates to direct the light beam onto the sample whose specular reflectance is to be measured. At the same time, the detector assembly is translated so as to maintain the same relationship with the input beam derived from the light source. The total length of the optical path remains the same so that the ratio of the two measurements provides the reflectance of the sample. A new reference measurement is required for each new angle of incidence as well as each different angle of polarisation.
Detectors having uniform sensitivity across their surface are required in such apparatus. Detectors not having uniform sensitivity require compensatory measures such as use of a scrambling light pipe to reduce sensitivity of the apparatus to misalignment of the beam of light. Moreover, most detectors are limited to a particular range of wavelengths of light. As a result, in order to cover a broad range of wavelengths it is necessary to employ multiple detectors, each detector catering for a portion of the desired wavelength range. This substantially increases the complexity of the apparatus since means for switching between the detectors must be provided in the form of a mechanical movement of each detector, or a mechanical movement of an optical component to shift the beam of light. These types of mechanisms invariably add bulk and complexity to the apparatus.
Other arrangements involve an integrating sphere to scramble the beam of light and reduce sensitivity to misalignment of the beam of light and/or non uniform sensitivity of the detector employed. An integrating sphere can accommodate multiple detectors and does not require mechanical changes between wavelength ranges. However, the application of integrating spheres is limited to restricted wavelength ranges and significantly reduces signal levels.
Other issues with apparatus for measuring absolute specular reflectance include that every specular reflectance at least partially polarises the reflected beam. The amount of polarisation is dependent on the nature of the sample and the angle of incidence. Accordingly, in order to accurately measure the spectral reflectance of a sample, it is necessary to measure both the intensity and the polarisation of the reflected light.
Accordingly, it is recognised that the range of commercial spectrometer apparatus and accessories currently available have various limitations.
The discussion of the background to the invention hereinabove is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of the present application.
An object of the present invention is to provide relatively simple spectrometer apparatus and accessory providing improved accuracy and efficiency by addressing at least one of the aforementioned limitations.